MACROSCOPIC ALGAL FOSSILS FROM THE CRYOGENIAN OF MONGOLIA

We describe putative fossil algae from carbonates of the ~715-635 Ma Tayshir member of the Tsagaan Oloom Formation, Mongolia. Macroscopic fossil material, consisting of 60 – 700 μm by 0.25 – 4.3 mm rigid sheets is present both in thin sections and in residue of carbonate macerations from three stratigraphic sections. Etched thin sections, macerated material, and TEM sections exhibit a pseudoparenchymatous structure expressed as polygonal cell wall structures 8-20 μm in maximum diameter. Many specimens show conspicuous raised warty surface structures ranging in height from 10 – 100 μm. Thin section specimens show evidence of transport before burial, and were rigid enough to behave as clasts. This observation suggests that they were cohesive before transport, and may have been biomineralized. FTIR spectroscopy shows that specimens consist of organic material and contain aromatic compounds and –OH and C-O bonds, as well as an aliphatic component, but their spectra differ from those previously published for Proterozoic fossils. Possible modern analogs of the fossil sheets are found in the red algae, including the coralline red algae, which often form macroscopic crusts composed of polygonal pseudoparenchymatous cells. Many modern forms exhibit warty reproductive structures (conceptacles), similar to protuberances seen in Tayshir fossils. A major difference between modern corallines and fossil specimens is the evidence for extensive calcification: modern corallines are calcified, but fossil specimens presented here lack evidence of this process. While the oldest known fossil interpreted as a coralline alga is Ordovician, molecular clock analyses and fossil evidence show that the roots of the greater red algal clade lay deep within the Neoproterozoic, raising the possibility that these fossils represent stem group red algae. The Tayshir fossils may represent the oldest evidence of complex macroscopic multicellularity in the fossil record and have implications for the evolution of the red algal clade and the dynamics of late Cryogenian ecosystems, which are otherwise dominated by microscopic organisms. In addition, these fossils add to the growing diversity of carbonate-hosted Neoproterozoic fossils, emphasizing the importance of a broad search image in Precambrian paleontology.